The present invention relates to an improved power/ground ball pad array on a printed circuit board (PCB) that is capable of reducing board-level IR drop in a microelectronic system.
An integrated circuit (IC) device typically includes an IC chip that is housed in a package. The IC chip typically includes a circuit fabricated by lithographically patterning conductive and insulating materials on a thin wafer of semiconductor using known fabrication techniques. The package supports and protects the IC chip and provides electrical connections between the circuit and an external circuit board. Several known package types are used to house IC chips, such as ball grid arrays (BGAs), pin grid arrays (PGAs), plastic leaded chip carriers, plastic quad flat packs and others, for example.
A ball grid array (BGA) package for higher performance applications is known in the art. A BGA package is typically joined to a mounting board by use of conductive balls (bumps) arranged in an array on its back as external terminals, instead of using a lead frame. Because the entire back surface of the semiconductor package can be used for connection to the board, the number of input/output pads can be markedly increased. To support higher functionality, the number of pins has remarkably increased.
As system complexity and operational speeds increase, the power consumption of integrated circuits increases dramatically. Additionally, the IC supply voltage continues to drop with the inevitable scaling of VLSI technology. Reducing the nominal supply voltage is accompanied by a reduction in device noise margins, making components more vulnerable to power supply noise. This noise consists of the dynamic AC voltage fluctuation due to the frequency dependent distributed parasitics inherent in today's power distribution systems, and the DC voltage drop (i.e., “IR” drop).
In a microelectronic system, the system's IR drop may be budgeted into three portions: on-chip, package and board. On-chip IR drop has been extensively studied because the resistive loss is severe due to the fine feature-size of the on-die power grid. On the other hand, package and board-level IR drop have not been given much attention. Due to increased current requirements and reduced supply voltage noise margins, package and board IR drop now can have a significant impact on the operation of high-speed devices.
Several factors contribute to increased off-chip path resistances. In multilayer IC packages such as BGAs, for example, the power distribution usually traverses multiple layers from the balls to the chip connect bumps. These paths are much shorter than those on the board; however, package power and ground planes usually require much more irregular shapes to accommodate the chip I/O breakout and usually are not allowed to fill an entire plane. Many packages also contain a number of power domains, but a very limited number of layers are available for their distribution.
Therefore, it is common for power distributions to contain complex shapes and other non-ideal routing. Printed circuit boards have their own share of issues as well. In large and complex PCBs, the power distribution system may have to traverse several feet of planes and traces to reach the far-end devices. Therefore, far-end devices will see a larger voltage drop. Because of the long distribution paths, it is possible for designs to deliver insufficient voltage to some devices. Therefore, for high-current and low-voltage designs, it is becoming critically important to include package and board IR drop into the total noise budget of the system.